1. Field of the Invention
The present invention relates to a vacuum fluorescent display apparatus, comprising a phosphor for a low-energy electron beam as a light source.
2. Description of the Related Art
Phosphors that are excited by low-energy electron beams and that are used in vacuum fluorescent display apparatuses include phosphors in which an oxygen atom is included in the chemical formula of the parent material. Examples of phosphors of such description include ZnO, Ca(1-x)SrxTiO3:Pr,M (where 0≦x≦1 and M is at least one type of metal selected from Al, Ga, In, Mg, Zn, Li, Na, and K), or Ln2O2S:Re (where Ln is at least one type of metal selected from La, Gd, Y, and Lu; and Re is Eu, Tb, or Sm). Phosphors of such description are prone to degradation in brightness caused by barium generated by evaporation from a filament cathode.
In particular, oxide-based red phosphors that do not contain Cd suffer from significant degradation in brightness in a voltage region of 30V or above. Therefore, a decrease in display quality due to degradation of a red phosphor occurs not only in instances in which only red phosphor is used, but also in instances in which a red phosphor is combined with a phosphor corresponding to another color and a mixture of colors is used. Accordingly, there is a significant decrease in the lifespan of a vacuum fluorescent display apparatus in which a red phosphor of such description are used.
Conventionally, the following methods have been proposed as methods for improving the lifespan characteristics of oxide-based red phosphors.
(1) A protective layer made from an oxide of Al, Ti, Si, Ga, Zn, Sn, Bi, or another element is formed on a phosphor comprising a parent material made from an alkaline earth metal and an oxide, e.g., a SrTiO3:Pr,Al phosphor, in order to prevent a degradation in brightness of the phosphor (JP-A 8-283709).
(2) TiO2 or ZnO2 is used in a supplementary manner as a non-evaporating getter material. This getter material removes residual gas in a vacuum container in an effective manner, thereby improving the lifespan characteristics of the vacuum fluorescent display apparatus (JP-A 2000-340140).
(3) A gas storage material containing ZrO, (where 1≦x≦2) is installed in a vacuum container. This gas storage material forms and maintains a vacuum in the vacuum container (JP-A 2005-209594).
According to method (1) above, the phosphor surface is protected from carbon, barium, or similar substances, thereby improving the lifespan characteristics. However, the lifespan of a vacuum fluorescent display apparatus made using an oxide-type red phosphor treated as described above remained extremely low compared to that of ZnCdS-based phosphors. Also, in order to extend the lifespan of the vacuum fluorescent display apparatus, it is necessary to increase the amount of protective film adhering to the phosphor. However, increasing the amount of protecting film adhered onto the surface of an oxide-based red phosphor results in a decrease in brightness.
Also, according to methods (2) and (3) described above, a getter material is installed in a vacuum container in order to enhance the level of vacuum in the vacuum container. The lifespan characteristics and light-emitting characteristics of the vacuum fluorescent display apparatus are improved by increasing the level of vacuum in the vacuum container using the getter material. Getter materials of such description is merely subjected to heat treatment and activated during manufacture. For example, TiO2 is changed to TiO or Ti, thereby promoting adsorption of O and H in the vacuum container.
However, when the vacuum fluorescent display apparatus is being driven, the heat treatment is not applied, and the getter is not bombarded with an electron beam. Therefore, getters of such description are not activated. Accordingly, no adsorption of gas in the tube generated during driving takes place; in particular, no adsorption of barium, which is harmful to the phosphor, takes place. As a result, the lifespan of the vacuum fluorescent display apparatus cannot be enhanced.